Remove supervised user import command line flags.

This CL removes the allow-create-existing-managed-users and the
disable-create-exsting-managed-users flags.

BUG=348925

Review URL: https://codereview.chromium.org/227543007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263255 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed